This invention relates to processes, continuous processes, cured composites and related compositions. One embodiment of the invention particularly relates to a process, preferably a continuous process for producing a light pipe with flexibility or a flexible light pipe ("FLP"), and the improved FLP product which the process produces. The invention also relates to efficiently fabricating a FLP having a core diameter of at least 3 millimeters, useful for conveying visible light and which remains flexible and transparent under a wide range of use conditions.
The production of a cured composite is known from European Patent Application 469 673, in which a continuous object of thermoset polymer is made by continuous polymerization, utilizing ultraviolet radiation to polymerize monomer in a clad. This production method teaches the use of ultraviolet (UV) radiation to obtain polymerization within a few seconds. Problems with this method include: 1) reduction of light transmittance and increased yellowness in thermoset polymers due to use of UV initiators, 2) requirement of fast polymerization such as within a few seconds by UV radiation, to avoid shrinkage of the thermoset polymer within the clad after the clad has solidified, and 3) the teaching away from thermal cure by heat. Further, this reference does not teach a light pipe having flexibility and optical properties such as low light transmission loss.
The range of applications to which a visible light-conveying light pipe can be applied are dictated by the physical and optical characteristics the FLP develops during the fabrication process. For example, in U.S. Pat. No. 4,708,833 a method teaches rapidly crosslinking ("curing") a falling strand of polymer syrup by ultraviolet light, electron beam, or heat, thus curing the polymer syrup (elastomeric core) and achieving structural integrity, and then simultaneously or subsequently applying a cladding. Production methods using such rapid curing of the core suffer from several disadvantages: 1) limited breadth of available curing chemistry, 2) increased production rates require a proportional increase in cure rate or distance to a take-up roller, and 3) reduced light transmittance of the light pipe, particularly in the blue portion of the visible spectrum (i.e. increased yellowness) from use of curing materials that degrade or yellow upon heating or irradiation.
U.S. Pat. No. 4,957,347 ("347") teaches a flexible, clad monofilament production method which fills a pre-formed cladding tube with monomer, and then cures the monomer within the filled cladding. Such monomer-fill processes have advantages in providing a range of composition, curing chemistries and filling rates which can be employed, but suffer from four major limitations: 1) heat-transfer problems during polymerization and curing, 2) volumetric shrinkage upon polymerization, 3) thin claddings which require reinforcement (for example, a thin clad of poly(tetrafluoroethylene)) with another tube layer (e.g. a "sheathing" layer), and 4) relatively low production rates. Problems such as poor heat-transfer rates, low boiling point of many useful monomers, and the need for practical production rates, led to the use of large, complex, pressurized polymerization equipment, as a partial solution to some of the four above identified problems. Problems such as volumetric shrinkage in monomer-fill processes led to the use of short monomer-filled cladding sections processed in U-shaped tubes or by slow feeding of monomer-filled cladding through a heated bath ("bulk polymerization methods"), as a partial solution to the above identified problems. However, these slow laborious bulk production methods led to the use of high initiator levels to speed up production rates. These high initiator levels contribute to light absorption losses when heating such products in air. Further, bulk polymerization methods suffer from polymer compositional drift during copolymerization which can lead to phase separation and concomitant haze, thereby reducing light transmission efficiency. Bulk polymerization, as practiced in '347, constrains the range of useful comonomer combinations. Neither the use of pressurized polymerization equipment nor carefully designed copolymerization temperature profiles (methods which individually solve some deficiencies) collectively overcome all the four above identified problems in these methods.
Based on, but not limited to, the above identified problems in the art, a continuous and efficient process has not been developed prior to this invention, to manufacture a light pipe having features including ambient temperature flexibility, good transmittance, low yellowness, softness, and property retention under thermal aging conditions. It is contemplated that the diameter of light pipes of the invention shall exceed 1 millimeter (mm) and shall typically range from 3 to 20 mm. It would be a significant advancement of the art to have a method for producing a light pipe and a light pipe product which achieves these features. It would be particularly advantageous to be able to cure a core under thermally controlled conditions, thereby avoiding the constraints of "rapid curing" by UV radiation in a continuous process. It would be generally advantageous to decouple the light pipe production step from structural integrity and cure rate requirements of the light pipe core. It also would be advantageous to decouple a sheathing step, if desired from the process while using thin-walled cladding materials. It would also be advantageous to have a process generally applicable to producing cured composites from a variety of compositions having a variety of end uses.